[Numpy-discussion] question about long doubles on ppc64el

[Thomas Caswell]

Folks,

Over at h5py we are trying to get a release out and have discovered (via
debian) that on ppc64el there is an apparent disagreement between the size
of a native long double according to hdf5 and numpy.

For all of the gorey details see: https://github.com/h5py/h5py/issues/817 .

In short, `np.longdouble` seems to be `np.float128` and according to the
docs should map to the native 'long double'. However, hdf5 provides a
`H5T_NATIVE_LDOUBLE` which should also refer to the native 'long double',
but seems to be a 64 bit float.

Anyone on this list have a ppc64el machine (or experience with) that can
provide some guidance here?

Tom

[Jens Nielsen]

According to
https://docs.scipy.org/doc/numpy-dev/user/basics.types.html#extended-precision
numpy
long doubles are typically zero padded to 128 bits on 64 bit systems could
that be the reason?

Post by Thomas Caswell
Folks,
Over at h5py we are trying to get a release out and have discovered (via
debian) that on ppc64el there is an apparent disagreement between the size
of a native long double according to hdf5 and numpy.
For all of the gorey details see: https://github.com/h5py/h5py/issues/817 .
In short, `np.longdouble` seems to be `np.float128` and according to the
docs should map to the native 'long double'. However, hdf5 provides a
`H5T_NATIVE_LDOUBLE` which should also refer to the native 'long double',
but seems to be a 64 bit float.
Anyone on this list have a ppc64el machine (or experience with) that can
provide some guidance here?
Tom
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[Charles R Harris]

Post by Thomas Caswell
Folks,
Over at h5py we are trying to get a release out and have discovered (via
debian) that on ppc64el there is an apparent disagreement between the size
of a native long double according to hdf5 and numpy.
For all of the gorey details see: https://github.com/h5py/h5py/issues/817 .
In short, `np.longdouble` seems to be `np.float128` and according to the
docs should map to the native 'long double'. However, hdf5 provides a
`H5T_NATIVE_LDOUBLE` which should also refer to the native 'long double',
but seems to be a 64 bit float.
Anyone on this list have a ppc64el machine (or experience with) that can
provide some guidance here?

I believe the ppc64 long double is IBM double double, i.e., two doubles for
128 bits. It isn't IEEE compliant and probably not very portable. It is
possible that different compilers could treat it differently or it may be
flagged to be treated in some specific way.

Chuck

[Matthew Brett]

Hi,

Post by Thomas Caswell
Folks,
Over at h5py we are trying to get a release out and have discovered (via
debian) that on ppc64el there is an apparent disagreement between the size
of a native long double according to hdf5 and numpy.
For all of the gorey details see: https://github.com/h5py/h5py/issues/817 .
In short, `np.longdouble` seems to be `np.float128` and according to the
docs should map to the native 'long double'. However, hdf5 provides a
`H5T_NATIVE_LDOUBLE` which should also refer to the native 'long double',
but seems to be a 64 bit float.
Anyone on this list have a ppc64el machine (or experience with) that can
provide some guidance here?

I know that long double on numpy for the PPC on Mac G4 (power64 arch)
is the twin double, as expected, so I'd be surprised if that wasn't
true for numpy on ppc64el .

Do you want a login for the G4 running Jessie? If so, send me your
public key off-list?

Cheers,

Matthew